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RESEARCH PRODUCT

Temperature and angular dependence of the upper critical field of UPd2Al3 thin films

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Abstract Temperature and angular resolved measurements of the upper critical field Bc2 of UPd2Al3 thin films are presented and compared with new theoretical calculations for weak-coupling s- and d-wave superconductors in the clean limit. In order to take account of the observed anisotropy of the initial slopes near T c , B′ c 2 = −4.9 T/K for B ‖ c and B′ c 2 = −5.8 T/K for B ⊥ c , and the crossing of the phase boundaries in the temperature dependence of Bc2 near T ∗ ≈ 0.75 T/T c , the calculations include anisotropic orbital and paramagnetic pair breaking. Recent speculations upon the occurence of a non-uniform superconducting “Fulde-Ferrell-Larkin-Ovchinnikov”-state are also considered and incorporated in the calculations. In the region around T ∗ a minimum between B ‖ c and B ⊥ c in the out-of-plane angular dependence of the upper critical field Bc2(Θ) is found experimentally. This cannot be explained by an s-wave order parameter and reduces drastically the possible d-wave functions. To explain the overall data, a mixing of s-wave and a particular d-wave order parameter is proposed.